Implemented --flake flag for nix build

Also fixed Eelco's PR comments

src/nix/build.cc

@@ -1,3 +1,5 @@
+#include "primops/flake.hh"
+#include "eval.hh"
 #include "command.hh"
 #include "common-args.hh"
 #include "shared.hh"
@@ -9,6 +11,8 @@ struct CmdBuild : MixDryRun, InstallablesCommand
 {
     Path outLink = "result";
 
+    std::optional<std::string> flakeUri = std::nullopt;
+
     CmdBuild()
     {
         mkFlag()
@@ -22,6 +26,11 @@ struct CmdBuild : MixDryRun, InstallablesCommand
             .longName("no-link")
             .description("do not create a symlink to the build result")
             .set(&outLink, Path(""));
+
+        mkFlag()
+            .longName("flake")
+            .description("update lock file of given flake")
+            .dest(&flakeUri);
     }
 
     std::string name() override
@@ -52,6 +61,8 @@ struct CmdBuild : MixDryRun, InstallablesCommand
     {
         auto buildables = build(store, dryRun ? DryRun : Build, installables);
 
+        auto evalState = std::make_shared<EvalState>(searchPath, store);
+
         if (dryRun) return;
 
         for (size_t i = 0; i < buildables.size(); ++i) {
@@ -66,6 +77,10 @@ struct CmdBuild : MixDryRun, InstallablesCommand
                         store2->addPermRoot(output.second, absPath(symlink), true);
                     }
         }
+
+        if (flakeUri) {
+            updateLockFile(*evalState, *flakeUri);
+        }
     }
 };
 

src/nix/command.cc

@@ -27,15 +27,6 @@ void StoreCommand::run()
     run(getStore());
 }
 
-JsonFormattable::JsonFormattable()
-{
-    mkFlag()
-        .longName("json-formattable")
-        .shortName('j')
-        .description("output will be printed as json")
-        .handler([&]() { jsonFormatting = true; });
-}
-
 StorePathsCommand::StorePathsCommand(bool recursive)
     : recursive(recursive)
 {

src/nix/command.hh

@@ -26,13 +26,6 @@ private:
     std::shared_ptr<Store> _store;
 };
 
-struct JsonFormattable : virtual Command
-{
-    bool jsonFormatting = false;;
-
-    JsonFormattable();
-};
-
 struct Buildable
 {
     Path drvPath; // may be empty

src/nix/flake.cc

@@ -34,7 +34,28 @@ struct CmdFlakeList : StoreCommand, MixEvalArgs
     }
 };
 
-struct CmdFlakeInfo : FlakeCommand, JsonFormattable
+struct CmdFlakeUpdate : FlakeCommand
+{
+    std::string name() override
+    {
+        return "update";
+    }
+
+    std::string description() override
+    {
+        return "update flake lock file";
+    }
+
+    void run(nix::ref<nix::Store> store) override
+    {
+        auto evalState = std::make_shared<EvalState>(searchPath, store);
+
+        if (flakeUri == "") flakeUri = absPath("./flake.nix");
+        updateLockFile(*evalState, flakeUri);
+    }
+};
+
+struct CmdFlakeInfo : FlakeCommand, MixJSON
 {
     std::string name() override
     {
@@ -50,7 +71,7 @@ struct CmdFlakeInfo : FlakeCommand, JsonFormattable
     {
         auto evalState = std::make_shared<EvalState>(searchPath, store);
         nix::Flake flake = nix::getFlake(*evalState, FlakeRef(flakeUri));
-        if (jsonFormatting) {
+        if (json) {
             nlohmann::json j;
             j["location"] = flake.path;
             j["description"] = flake.description;
@@ -65,7 +86,9 @@ struct CmdFlakeInfo : FlakeCommand, JsonFormattable
 struct CmdFlake : virtual MultiCommand, virtual Command
 {
     CmdFlake()
-        : MultiCommand({make_ref<CmdFlakeList>(), make_ref<CmdFlakeInfo>()})
+        : MultiCommand({make_ref<CmdFlakeList>()
+            , make_ref<CmdFlakeInfo>()
+            , make_ref<CmdFlakeUpdate>()})
     {
     }